Method for determining an end of discharge voltage for a secondary battery

ABSTRACT

Method for determining a determination voltage (end of discharge voltage) of a secondary battery having a nickel-hydroxide positive electrode based on the state of charge of the battery. The end of discharge voltage is determined in consideration of the voltages when the state of charge reaches a permissible lower limit after first and subsequent discharges of the battery. The voltage at which the state of charge reaches the lower limit for discharges after the first discharge gradually decreases due to a memory effect. As a result, the end of discharge voltage is lower than the voltage at which the state of charge reaches the lower limit after the first discharge.

BACKGROUND OF THE INVENTION

The present invention relates to a method of detecting remainingcapacity for secondary batteries wherein nickel hydroxide is used as apositive electrode active material, such as a nickel metal-hydridesecondary battery and a nickel cadmium secondary battery.

In appliances in which secondary batteries are used as a power source,it is necessary to detect the remaining capacity of the secondarybatteries in order to prevent overdischarge and overcharge, so that theappliances are used within a suitable range of the remaining capacity ofthe batteries.

The remaining capacity of a secondary battery can be indexed by thestate of charge (SOC) which indicates the rate of accumulated electricalquantities in relation to battery capacity. This SOC can be determinedfrom voltage because charge/discharge characteristic curvescorresponding to charge/discharge electric current and temperature canbe determined through experimentation. That is, by detecting voltage ata certain electric current value and temperature, and by utilizing thecharge/discharge characteristic curves corresponding to the aboveelectric current value and temperature, the SOC of the battery can beobtained.

Accordingly, it is possible to detect, by measuring voltage of asecondary battery, whether the SOC after a final discharge (i.e., thelast discharge before the battery is recharged) has reached apermissible lower limit (an SOC of 20%, for example). A determination isthen made in accordance with the results whether further discharge willinduce an overdischarged state, whereupon discharge can be stopped andthe battery charged.

The determination voltage (also referred to as end of discharge voltage)for determining whether the SOC has reached a permissible lower limit asdescribed above has usually been determined in the prior art as follows.The determination voltage was determined based on a dischargecharacteristic curve (complete discharge characteristic curve)indicative of the relationship between the SOC and voltage that isobtained at the time of complete discharge (first discharge). By way ofexample, in the case of the discharge characteristic curve A shown bysolid lines in FIG. 1, the voltage value 1.237V that corresponds to theSOC of 20% was defined to be the determination voltage for determiningwhether the SOC has reached the permissible lower limit.

However, in the case of nickel metal-hydride or nickel-cadmium secondarybatteries using nickel hydroxide as a positive electrode material, asthe charge and incomplete discharge cycles are repeated, a memory effectis observed wherein, as shown in the discharge characteristic curves inFIGS. 2 and 3, voltage corresponding to the SOC decreases in accordancewith the number of charge/discharge cycles (represented by [1]˜[10]),and drops considerably, particularly at the final discharge. Thus, ifthe determination voltage is defined based on the complete dischargecharacteristic curve as above, there is a problem that it may beerroneously determined that the SOC has reached the permissible lowerlimit (for example, an SOC of 20%) although there actually remains asufficient amount of remaining capacity (an SOC of 30%, for example) inthe case where charge and discharge are repeated a large number of timesas shown in FIG. 3.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of detectingthe determination or end of discharge voltage of secondary batteriesusing a nickel-hydroxide positive electrode, by which a correctdetermination of whether the SOC has reached a permissible lower limiteven after the final discharge is possible by measuring the voltage ofthe batteries.

In order to achieve the above object, the method of detecting thedetermination voltage based on the SOC of a secondary battery which usesa nickel-hydroxide positive electrode according to a first aspect of thepresent invention includes the steps of setting a permissible lowerlimit of the SOC designed, for example, to prevent overdischarge of thebattery, and determining the voltage of the battery when the SOC reachesthe lower limit after multiple discharges, the voltage for dischargesafter the first discharge gradually decreases from the voltage after thefirst discharge due to a memory effect. Thereafter, the voltage fordetermination, i.e., the voltage at which discharge should be stoppedand charging commenced, is determined in consideration of the voltagesat which the SOC is the set lower limit after the multiple dischargeswhereby the determination voltage is lower than the voltage the batteryreaches after the first discharge for the lower limit of the SOC, and adetermination is made whether the SOC has reached the lower limit basedon this voltage for determination, which entails a comparison of theactual voltage to the determination voltage.

According to the above method, the determination voltage is definedlower than the voltage when the SOC reaches the permissible lower limitafter the first discharge in consideration of a memory effect observedin secondary batteries using a nickel-hydroxide positive electrode. As aresult, even in the case of repeating charge and discharge cycles alarge number of times, the actual SOC when it is determined to havereached the permissible lower limit can be made closer to the lowerlimit of the SOC defined as the determination reference.

It is preferable to set the voltage for determination 0.01˜0.10V/celllower than the voltage when the SOC reaches the permissible lower limitat the first discharge.

Also, to achieve the above object, the method of detecting thedetermination voltage based on the SOC of a secondary battery which usesa nickel-hydroxide positive electrode according to a second aspect ofthe present invention includes the steps of setting a permissible lowerlimit of the SOC designed, for example, to prevent overdischarge of thebattery, and determining the voltage of the battery when the SOC reachesthe lower limit after multiple discharges, the voltage for dischargesafter the first discharge gradually decreases from the voltage after thefirst discharge due to a memory effect. Several determination voltagesare determined from the voltages at which the SOC is the set lower limitafter the discharges and thus the determination voltage of the batteryis variable in accordance with the number of times of charge anddischarge of the battery.

According to the above method, the voltage for determination is definedin accordance with the number of charge and discharge cycles repeated inconsideration of a memory effect observed in secondary batteries using anickel-hydroxide positive electrode. As a result, at any time ofdischarge, the actual SOC when it is determined to have reached thepermissible lower limit can be made remarkably closer to the lower limitof the SOC defined as the determination reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing to a larger scale a part of dischargecharacteristic curves indicative of the relationship between the SOC andvoltage at the time of discharge;

FIG. 2 is a graph showing discharge characteristic curves indicative ofthe relationship between the SOC and voltage at the time of discharge;and

FIG. 3 is a graph showing charge and discharge characteristic curvesindicative of the relationship between the SOC and voltage at the timeof charge and discharge.

DETAILED DESCRIPTION

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings for theunderstanding of the present invention. The method of determining thedetermination voltage based on the SOC in accordance with the presentinvention is applied to a nickel metal-hydride secondary battery for useas a power supply for driving an electric vehicle or a hybrid vehiclewhich uses a motor and engine in combination for a drive force, and isimplemented for the purposes of controlling charge and discharge cyclesin order to maintain the nickel metal-hydride secondary battery within asuitable range of application without falling into an overdischargestate.

In secondary batteries that use nickel hydroxide as a positive electrodematerial such as the above mentioned nickel metal-hydride secondarybattery, as charge and discharge cycles are repeated in which thedischarge of the battery is not complete, a memory effect is observedwherein the voltage decreases from the value at the time of firstdischarge in accordance with the number of charge/discharge cycles, ascan be seen from the discharge characteristic curves indicative of therelationship between the SOC and voltage in FIG. 2. Normally, charge anddischarge of secondary batteries are controlled such that discharge isstopped when the SOC at the final discharge, i.e., the last dischargebefore charging begins, is detected to have reached a permissible lowerlimit (an SOC of 20%, for example) and switched to a charging state, inorder to prevent damage to cells caused by overdischarge. Under suchcondition, a complete discharge is inhibited, as a result of which theoccurrence of the above mentioned memory effect is inevitable.

The above mentioned discharge characteristic curves vary depending ondischarge electric current and battery temperature. A number ofdischarge characteristic curves corresponding to each one of dischargeelectric currents and battery temperatures are thus preliminarilyobtained.

FIG. 2 shows discharge characteristic curves corresponding to a certaindischarge electric current and battery temperature, obtained throughexperimentation, including not only the one at the first discharge(which coincides with the complete discharge characteristic curve inthis case), but also the discharge characteristic curves at the secondto tenth time of repeating charge and discharge ([1]˜[10] in FIG. 2represent the number of times of charge and discharge). When theallowable lower limit of the SOC at the final discharge is set to be20%, the voltage corresponding to the above lower limit decreases as thenumber of times of charge and discharge increases, as shown in Table 1below.

TABLE 1 Charge/discharge number of Voltage when the SOC times is 20% 11.237V V_(o1) 2 1.222V V_(o2) 3 1.212V V_(o3) 4 1.205V V_(o4) 5 1.197VV_(o5) 6 1.190V V_(o6) 7 1.186V V_(o7) 8 1.180V V_(o8) 9 1.175V V_(o9)10 1.169V V_(o10)

Accordingly, if the voltage for determination were to be determinedbased on the discharge characteristic curve at the first discharge(which coincides with the complete discharge characteristic curve) as inthe prior art, it would be 1.237V under a condition that the permissiblelower limit of the SOC is set at SOC 20%. However, it is irrational touse this value of 1.237V as the voltage for determination, in the lightof the above mentioned memory effect. The voltage, for example,according to the discharge characteristic curve at the tenth dischargecorresponding to an SOC of 20% is 1.169V, and there is a greatdifference between this and the voltage for determination 1.237V.

FIG. 1 shows the discharge characteristic curve A at the first dischargeand the discharge characteristic curve B at the tenth discharge.Provided that the voltage for determination when the SOC is 20% is setto be 1.237V as in the prior art, it will be correctly determined thatthe SOC has reached the allowable lower limit where the SOC is 20%(shown by Q₁) after the first discharge, but after the tenth discharge,it will be erroneously determined, when the SOC is still 30% (shown byQ₁₀), that the SOC has already reached the allowable lower limit.

In view of the above, in this embodiment, in determining the abovevoltage for determination, not only the voltage value (V₁) when the SOCreaches the allowable lower limit after the first discharge, but alsoother voltage values (V₁₀ and others) when the SOC reaches the allowablelower limit after subsequent discharges are taken into consideration,and thus the voltage for determination is set lower (V₀) than thevoltage (V₁) after the first discharge. Specifically, in the case shownin FIG. 1, the voltage for determination (V₀) is set to be 1.210V. Inthis way, when it is determined that the SOC has reached the allowablelower limit at the first discharge, the actual value of the SOC (P1)will be 16˜17%. Meanwhile, when it is determined that SOC has reachedthe allowable lower limit at the tenth discharge, the actual value ofthe SOC (P₁₀) will be 23˜24%, so, in either case, there will be a smalldiscrepancy in relation to the reference value of the SOC of 20%. It isdesirable that there is only ±5% discrepancy; FIG. 1 shows the casewherein the voltage for determination (V₀) is set appropriately so thatthe discrepancy is kept within the range of ±5%.

It is further preferable to set the above voltage for determination (V₀)0.01˜0.010V per one cell lower than the voltage value (V₁) when the SOCreaches the permissible lower limit after the first discharge.

Decreasing rate of voltage of the discharge characteristic curve causedby the memory effect reduces as the charge and discharge are repeatedmany times, and there is not a large difference, for example, betweenthe decreasing rate at the twentieth discharge and that at the tenthdischarge. Accordingly, discharge characteristic curves after the firstto, approximately, tenth discharge will suffice to provide a data, basedon which the above voltage for determination (V₀) is determined.

In this embodiment, a determination is made whether the SOC of thesecondary battery has reached a permissible lower limit, using thevoltage (V₀) for determination determined as described above. Dischargeelectric current, battery temperature, and voltage (V) of the secondarybattery to be determined are first measured, the voltage fordetermination (V₀) which corresponds to the obtained discharge electriccurrent and battery temperature is selected and compared with thevoltage (V), and when V≦V₀ it is ascertained that the SOC has reachedthe allowable lower limit.

Next, another embodiment of the present invention will be described. Inthis embodiment, the above voltage for determination (V₀) is a variable,so that it is suitably determined in accordance with the number of timesof charge and discharge. In this way, the actual value of the SOC whenit is determined to have reached the allowable lower limit at any timeof discharge can be made even closer to the value of the lower limit ofthe SOC (an SOC of 20%, for example) which is used as the determinationreference.

By way of example, in the case shown in FIG. 2 and Table 1, if thevalues denoted V₀₁˜V₀₁₀ in Table 1 are used as the voltage fordetermination in accordance with the number of times of charge anddischarge, the actual value of the SOC when it is determined to havereached the permissible lower limit and the lower limit of the SOC (anSOC of 20%) which is the determination reference can be made almost thesame.

The voltage for determination may be determined corresponding to eachtime of charge and discharge as in the case shown in Table 1, but it canalso be determined in steps by dividing the number of times of chargeand discharge into groups. For example, the voltage for determinationcan be determined as shown in the following Table 2 (V₀₁˜V₀₁₀)

TABLE 2 Charge/discharge number of Voltage for times determination 1 V₀₁2, 3 V₀₂₃ 4-6 V₀₄₆ 7-9 V₀₇₉ 10 or more V₀₁₀

In this embodiment, as described above, a determination is made whetherthe SOC of the secondary battery has reached a permissible lower limit,using the voltage for determination determined in accordance with thenumber of times of charge and discharge as described above. At the timeof Nth discharge, the discharge electric current, battery temperature,and voltage (V) of the secondary battery to be determined are measured,the voltage for determination (V_(ON)) which corresponds to the measureddischarge electric current and battery temperature as well as to the Nthtime of discharge is selected and compared with the voltage (V), andwhen V≦V_(ON) it is determined that the SOC has reached the allowablelower limit.

As set forth above, according to the present invention, whether the SOCof a secondary battery which uses a nickel-hydroxide positive electrodehas reached a permissible lower limit at the final discharge can becorrectly determined by measuring the voltage of the battery, whereforethe present invention is useful in preventing overdischarge andovercharge of a battery and in precisely controlling charge anddischarge of a battery.

What is claimed is:
 1. A method of determining an end of dischargevoltage of a secondary battery which has a positive electrode ofnickel-hydroxide based on a state of charge (SOC) of the battery,comprising the steps of: setting an allowable lower limit of the SOC ofthe battery; determining a voltage at which the SOC is the set lowerlimit after a first discharge of the battery; determining voltages atwhich the SOC is the set lower limit after subsequent discharges of thebattery after recharge of the battery, wherein the voltage at which theSOC is the set lower limit for discharges after the first dischargegradually decreases in view of a memory effect of the battery; anddetermining the end of discharge voltage in consideration of thevoltages at which the SOC is the set lower limit after the first andsubsequent discharges whereby the end of discharge voltage is lower thanthe voltage at which the SOC is the set lower limit after the firstdischarge.
 2. The method according to claim 1, further comprising thestep of setting the end of discharge voltage from 0.01 to about 0.10Vlower than the voltage at which the SOC is the set lower limit after thefirst discharge.
 3. The method according to claim 1, further comprisingthe steps of: determining an actual SOC after the first and subsequentdischarges of the battery; setting a permissible range of deviationbetween the actual SOC of the battery after the first and subsequentdischarges and the set lower limit of the SOC; and changing the end ofdischarge voltage if any of the actual SOC of the battery after thefirst and subsequent discharges is outside of the permissible range ofdeviation.
 4. The method according to claim 3, wherein the permissiblerange of deviation is set at ±5%.
 5. The method according to claim 1,wherein the step of setting the allowable lower limit of the SOC of thebattery comprises the step of setting the lower limit of the SOC to be20%.
 6. The method according to claim 1, wherein the step of determiningthe voltage at which the SOC is the set lower limit after subsequentdischarges of the battery comprises the step of determining the voltageat which the SOC is the set lower limit after nine sequential dischargesof the battery.
 7. A method of determining an end of discharge voltageof a secondary battery which has a positive electrode ofnickel-hydroxide based on a state of charge (SOC) of the battery,comprising the steps of: setting an allowable lower limit of the SOC ofthe battery; determining a voltage at which the SOC is the set lowerlimit after a first discharge of the battery; determining voltages atwhich the SOC is the set lower limit after subsequent discharges of thebattery after recharge of the battery, wherein the voltage at which theSOC is the set lower limit for discharges after the first dischargegradually decreases in view of a memory effect of the battery; anddetermining a plurality of different end of discharge voltages inconsideration of the voltages at which the SOC is the set lower limitafter the first and subsequent discharges such that the end of dischargevoltage varies based on the number of times of charge and discharge ofthe battery.
 8. The method according to claim 7, further comprising thestep of associating each of the end of discharge voltages with at leastone discharge of the battery.
 9. The method according to claim 7,further comprising the step of associating at least one of the end ofdischarge voltages with a plurality of discharges of the battery. 10.The method according to claim 7, wherein the step of setting theallowable lower limit of the SOC of the battery comprises the step ofsetting the lower limit of the SOC to be 20%.
 11. The method accordingto claim 7, wherein the step of determining the voltage at which the SOCis the set lower limit after subsequent discharges of the batterycomprises the step of determining the voltage at which the SOC is theset lower limit after nine sequential discharges of the battery.
 12. Amethod for increasing the efficiency of a rechargeable battery whilepreventing overdischarge of the battery, comprising the steps of:setting an allowable lower limit of a state of charge (SOC) of thebattery; determining voltages at which the SOC is the set lower limitafter a plurality of sequential discharges of the battery, the batterybeing recharged between discharges, wherein the voltages at which theSOC is the set lower limit for discharges after a first one of thedischarges are less than the voltage at which the SOC is the set lowerlimit after the first discharge in view of a memory effect of thebattery; determining an end of discharge voltage in consideration of thevoltages at which the SOC is the set lower limit after the plurality ofsequential discharges, the determination voltage being lower than thevoltage at which the SOC is the set lower limit after the firstdischarge; and stopping discharge of the battery when the voltagereaches the end of discharge voltage.
 13. The method according to claim12, further comprising the step of setting the end of discharge voltagefrom 0.01V to about 0.10V lower than the voltage at which the SOC is theset lower limit after the first discharge.
 14. The method according toclaim 12, further comprising the steps of: determining an actual SOCafter the discharges of the battery; setting a permissible range ofdeviation between the actual SOC after the discharges and the set lowerlimit of the SOC; and changing the end of discharge voltage if theactual SOC of the battery after any of the discharges is outside of thepermissible range of deviation.
 15. The method according to claim 14,wherein the permissible range of deviation is set at ±5%. 16.The methodaccording to claim 12, wherein the battery is a secondary battery whichhas a positive electrode of nickel-hydroxide.
 17. The method accordingto claim 12, wherein the step of setting the allowable lower limit ofthe SOC of the battery comprises the step of setting the lower limit ofthe SOC to be 20%.
 18. The method according to claim 12, wherein thestep of determining the voltages at which the SOC is the set lower limitafter the plurality of sequential discharges of the battery comprisesthe step of determining the voltages at which the SOC is the set lowerlimit after ten sequential discharges of the battery.